Summary Abstract The long-term goal of this project is to improve the outcome of surgical procedures involving skeletal muscle transfer, whether muscle transposition or transplantation. Under previous support from V.A. Rehab R&D, we characterized the design of muscles involved in tendon transfer surgery and developed high-resolution tools with which to study them. In this proposal, we exploit a relatively rare surgical procedure for brachial plexus injury, in which the gracilis muscle is surgically isolated and then transplanted into the arm to act as an elbow flexor. The key idea is that this surgical procedure allows us, for the first time, to completely characterize a single human skeletal muscle intraoperatively and then to predict and subsequently test its function in vivo. Further, because gracilis is the only muscle acting at the elbow we can explicitly test our model to optimize this and related types of surgery since no other muscles are involved in the elbow flexion movement. Our three aims are (1) to complete development of a fiber optic probe for measuring sarcomere length intraoperatively and then at one- and two-years postoperatively (2), to measure gracilis muscle sarcomere length and active and passive mechanical properties intraoperatively during surgical transplantation in 30 patients and (3) to compare predicted and actual function of the transferred gracilis muscle one and two years postoperatively. In the first aim, we will extend a very powerful optical imaging tool that we developed known as resonance reflection spectroscopy (RRS). This has been shown to measure muscle sarcomere length accurately but is sensitive to motion artifact. To solve this problem, we propose implementation of a photonic delay line enabling optical frequency domain interferometry (OFDI) to encode reflection spectra inside interferograms across many frequencies. Preliminary experiments show this is feasible but requires some refinement. The next two aims are interconnected. Aim 2 presents a sophisticated intraoperative experiment in which gracilis muscles are measured in vivo, in isolation, and then after transplantation into the arm. This aim is based on our previous intraoperative experience with tendon transfer surgery and biomechanical testing of muscle. In aim 3, using a deterministic model of muscle function (rather than current models which are indeterminate and must be solved by optimization), we will determine whether the typical biomechanical modeling approaches used in the field can accurately predict elbow flexion torque given the most detailed set of tissue-level parameters ever directly collected from a human muscle. If it is, this will be the first explicit validation of such an approach. If it is not, we will be able to identify and isolate the factor(s) that are obstacles to simulation validity. Successful completion of this project will improve our understanding of human skeletal muscle biomechanics, test our ability to model human joint function, and provide concrete surgical guidelines for this brachial plexus surgery.